Aspects and embodiments of the invention most generally pertain to an optical distance measuring apparatus and method and, more particularly, to a chip-scale, optical distance measuring apparatus comprising a photonic integrated circuit and associated coherent detection method.
Miniaturized imaging systems capable of high precision 3D range measurements are of great interest as sensors in applications such as industrial inspection, construction and architecture, Virtual Reality, and autonomous vehicles. Coherent laser ranging (LIDAR) offers improved resolution over RADAR for distance measurement and 3D imaging applications because 1) the shorter wavelength of light and the ability to sweep a focused beam over a target results in finer angular resolution; 2) the possibility of frequency modulating (FMCW) the laser through a large bandwidth provides finer range (distance) resolution.
Briefly, coherent, FMCW, optical detection provides a frequency modulated or chirped source light beam to a target from where it is reflected and collected onto a photodetector. The return or echo light beam is then mixed with a local oscillator (LO) light beam on the photodetector resulting in interference patterns that may be processed to provide detailed range and velocity information about the target. In a heterodyne configuration, the source of the echo and LO beams are often two independent lasers, with different emission frequencies. In a homodyne configuration a fraction of the source light beam is split off to form a local oscillator light beam, in effect forming an interferometer where the path length between the point of source beam emission and the target forms one branch, and the other consisting of the path of the LO light beam in the system. The resulting fringe patterns from the optical mixing of the two beams are manifest as a beat frequency tone on point detectors such as p-i-n photodetectors. Both configurations take advantage of the source and reflected light beam reciprocity, resulting in an improved signal-to-noise (SNR) ratio and high range measurement accuracy. The heightened SNR permits the use of much smaller receiver apertures, and shorter sampling times, enabling miniaturization and rapid measurement speed.
Coherent LIDAR devices for precision measurements over small distances have been described by Goodwin, U.S. Pat. No. 4,830,486, and Slotwinski and Kenyon, U.S. Pat. No. 4,824,251. Goodwin, for example, discloses frequency modulating a laser, splitting the beam into reference and target components, recombining the beams to create a beat signal (heterodyning) and determining properties of the beat wave by analyzing a pattern of fringes obtained on a detector. Both patents describe fiber optic embodiments of the method. However, systems capable of high precision measurements <1 cm over long ranges >20 meters at rapid rates (>100,000 measurements/second) are impractical due to limitations in digitization technologies. A major limitation of conventional coherent FMCW lidar is the time-bandwidth product of the chirp, which forces a tradeoff between measurement range, range resolution, and sampling time due to the limits in analog detection, modulation, and high-resolution digitization technologies. Current signal digitization technologies are limited in bandwidth to about 2 GHz, and systems capable of operating at the upper bandwidth limits are often impractically expensive.
Thus, what is needed is a practical, miniature, and inexpensive optical distance measurement system capable of great accuracy, rapid measurements, and large dynamic measurement range, that does not trade-off measurement range, range resolution, and measurement time. The present invention discloses such a system.